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1541 (250442) Red LED stays on, motor runs continuously.

JoeBliksem

New Member
Joined
Mar 3, 2022
Messages
5
Hi, I've been struggling with this board for weeks and it's just not coming right.

History
  • I had a spare working drive that I wanted to convert to a Meanwell switching PSU because the drive is 110v and I live in a 240v country.
  • The conversion worked and I was able to read and format upon my first tests.

IMG_20210728_143158.jpg
Ignore the large ICs in this image, I'm using all original MOS chips

  • I was a bit careless, trying to repair multiple drives, both Alps and Newtronics, and I suspect, at some point, I plugged in P8 wrong way around during subsequent testing. Either way, L8 went up in smoke.
  • I swapped L8 and C15 with good parts, the drive started up and initialised normally but it would no longer read or format (File not found)
  • I then swapped out all of the IC's related to FILE NOT FOUND as recommended by Ray Carlsen's guide.
This is the weird part:
  • After replacing UA1 and adding a socket, the drive no longer initializes normally and just spins continuously with the red LED on.
  • Even after returning the original UA1 it is still doing this. The original UA1 IC works in other drives.
  • I thought perhaps it's the socket, so I removed the socket and replaced it with a new one.
  • I can't see any damage or corrosion on the board.

  • I have since swapped out almost every IC on the board (except RAM and Motor Controller) and the problem persists.
  • I don't have an oscilliscope but I am getting odd multimeter readings e.g. the 6502 pin 40 goes to 3.8v instead of the expected 5v.
  • Reading 1.9v across C15 but getting 11.9v at C21

I'm pulling my hair out at this stage. Please help!
 
Last edited:
Hello
I would be more careful when handling these boards, they are more delicate than modern electronics. Either by chance/bad luck or me handling it without proper precautions reducing risk of ESD, an IC died when I was messing with the power supply of my 1541. See the lesson I learned here: https://forum.vcfed.org/index.php?t...ng-after-lubricating-1541-spindle-hub.1237433

If the voltages are wrong, start by checking the power supply with another, unrelated load (e.g. attaching a HDD is convenient for these PSUs). When it checks out, try to find out if a component on the board has shorted and is dragging down the supply rail. Do the measurements with the drive assembly once connected and once without.

You'll have to identify the faults one by one (by now there's probably more than one). Voltage measurements, visual inspection of the board for interrupted or bridged traces or hot/burned components is usually what I do.

Good luck!
 
I have two 1541 drives. The reason I got them was to run them in conjunction with an AIM-65 computer. I don't have a Commodore computer that connects to them.

Of course I investigated the design of the drive once I got them, and was very surprised that the entire design basically is a single board computer on its own, very sophisticated and quite compact.

This means that fault finding it, is an order of magnitude more difficult than a standard 5.25" floppy disk drive. In most of those cases the circuitry in a IBM style 5.25" drive is intuitive and amenable to fault finding sequential events on the scope.

I was "lucky" in that neither of my 1541 drives was defective, however, I figured if they developed a defect it would be significantly harder than usual to effect a repair, not impossible, just more difficult, because of the operation of the SBC.

powerlot has given good advice.
 
Hello
I would be more careful when handling these boards, they are more delicate than modern electronics. Either by chance/bad luck or me handling it without proper precautions reducing risk of ESD, an IC died when I was messing with the power supply of my 1541. See the lesson I learned here: https://forum.vcfed.org/index.php?t...ng-after-lubricating-1541-spindle-hub.1237433

If the voltages are wrong, start by checking the power supply with another, unrelated load (e.g. attaching a HDD is convenient for these PSUs). When it checks out, try to find out if a component on the board has shorted and is dragging down the supply rail. Do the measurements with the drive assembly once connected and once without.

You'll have to identify the faults one by one (by now there's probably more than one). Voltage measurements, visual inspection of the board for interrupted or bridged traces or hot/burned components is usually what I do.

Good luck!

Thank you, I was a bit of cowboy going into into these repairs, so you're probably right. I think the issue is somewhere in the board circuitry, fried diodes or the like. I'll figure out how to test beyond the IC's and see what happens.
 
I have two 1541 drives. The reason I got them was to run them in conjunction with an AIM-65 computer. I don't have a Commodore computer that connects to them.

Of course I investigated the design of the drive once I got them, and was very surprised that the entire design basically is a single board computer on its own, very sophisticated and quite compact.

This means that fault finding it, is an order of magnitude more difficult than a standard 5.25" floppy disk drive. In most of those cases the circuitry in a IBM style 5.25" drive is intuitive and amenable to fault finding sequential events on the scope.

I was "lucky" in that neither of my 1541 drives was defective, however, I figured if they developed a defect it would be significantly harder than usual to effect a repair, not impossible, just more difficult, because of the operation of the SBC.

powerlot has given good advice.
Yeah, I've fixed apple drives in the past and it was very simple. I had one drive from my childhood. I think I fried it trying to diagnose what turned out to be the typical Newtronics open head problem. Then I bought three more off of Ebay, untested and cheap. Two of those also had open heads. Luckily the third was an Alps unit and the mechanism is fine. Of the now four mainboards, two were working.

Following my PSU conversion, only one is working. I just can't live with the idea that I'm responsible for less Commodore stuff in the world, so must fix these!
 
  • I don't have an oscilliscope but I am getting odd multimeter readings e.g. the 6502 pin 40 goes to 3.8v instead of the expected 5v.
  • Reading 1.9v across C15 but getting 11.9v at C21

Have you tried to pull tantalum cap C15 out of the board and then checked the voltage of the +5V rail?
I've had a couple of drives with shorted C15. If shorted, it will most likely make one of the bridges (5V) very hot rather soon
after power has been switched on.

If the voltage level does not raise after pulling out C15, try replacing the 5V bridge. Both of those bridges do run hot
and I've also had failed bridges in the drives that I've managed to fix.

I believe you will need closer to +5V with major chips, ROMs and cpu for the drive to successfully boot. Now that
spindle is running indefinitely and red light is continuously on, the drive failes to boot. There are number of reasons
for this, but you must first make sure that chips have steady +5V or close. 4.8V might be sufficient for the drive to operate,
but I doubt 3,8V is not nearly close enough. 11.9V is probably good for the mechanics to work.

The drive will actually boot without C15 installed - if the pcb is otherwise in good condition. Of course, it is better
to have a good tantalum cap installed there, but this is still good to know for testing purposes.
 
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